Flow cytometry-based cell sorting was first introduced to the research community more than 20 years ago. It is a technology that has been widely applied in many areas of life science research, serving as a critical tool for those working in fields such as genetics, immunology, molecular biology and environmental science. Unlike bulk cell separation techniques such as immuno-panning or magnetic column separation, flow cytometry-based cell sorting instruments measure, classify and then sort individual cells or particles serially at rates of several thousand cells per second or higher. This rapid “one-by-one” processing of single cells has made flow cytometry a unique and valuable tool for extracting highly pure sub-populations of cells from otherwise heterogeneous cell suspensions.
Cells targeted for sorting are usually labeled in some manner with a fluorescent material. The fluorescent probes bound to a cell emit fluorescent light as the cell passes through a tightly focused, high intensity, light beam (typically a laser beam). A computer records emission intensities for each cell. These data are then used to classify each cell for specific sorting operations. Flow cytometry-based cell sorting has been successfully applied to hundreds of cell types, cell constituents and microorganisms, as well as many types of inorganic particles of comparable size.
Flow cytometers are also applied widely for rapidly analyzing heterogeneous cell suspensions to identify constituent sub-populations. Examples of the many applications where flow cytometry cell sorting is finding use include isolation of rare populations of immune system cells for AIDS research, isolation of genetically atypical cells for cancer research, isolation of specific chromosomes for genetic studies, and isolation of various species of microorganisms for environmental studies. For example, fluorescently labeled monoclonal antibodies are often used as “markers” to identify immune cells such as T lymphocytes and B lymphocytes, clinical laboratories routinely use this technology to count the number of “CD4 positive” T cells in HIV infected patients, and they also use this technology to identify cells associated with a variety of leukemia and lymphoma cancers.
Recently, two areas of interest are moving cell sorting towards clinical, patient care applications, rather than strictly research applications. First is the move away from chemical pharmaceutical development to the development of biopharmaceuticals. For example, the majority of new cancer therapies are bio-based. These include a class of antibody-based cancer therapeutics. Cytometry-based cell sorters can play a vital role in the identification, development, purification and, ultimately, production of these products.
Related to this is a move toward the use of cell replacement therapy for patient care. Much of the current interest in stem cells revolves around a new area of medicine often referred to as regenerative therapy or regenerative medicine. These therapies may often require that large numbers of relatively rare cells be isolated from sample patient tissue. For example, adult stem cells may be isolated from bone marrow and ultimately used as part of a re-infusion back into the patient from whom they were removed. Cytometry lends itself very well to such therapies.
There are two basic types of cell sorters in wide use today. They are the “droplet cell sorter” and the “fluid switching cell sorter.” The droplet cell sorter utilizes micro-droplets as containers to transport selected cells to a collection vessel. The micro-droplets are formed by coupling ultrasonic energy to a jetting stream. Droplets containing cells selected for sorting are then electrostatically steered to the desired location. This is a very efficient process, allowing as many as 90,000 cells per second to be sorted from a single stream, limited primarily by the frequency of droplet generation and the time required for illumination.
A detailed description of a prior art flow cytometry system is given in United States Published Patent Application No. US 2005/0112541 A1 to Durack et al.
Droplet cell sorters, however, are not particularly biosafe. Aerosols generated as part of the droplet formation process can carry biohazardous materials. Because of this, biosafe droplet cell sorters have been developed that are contained within a biosafety cabinet so that they may operate within an essentially closed environment. Unfortunately, this type of system does not lend itself to the sterility and operator protection required for routine sorting of patient samples in a clinical environment.
The second type of flow cytometry-based cell sorter is the fluid switching cell sorter. Most fluid switching cell sorters utilize a piezoelectric device to drive a mechanical system which diverts a segment of the flowing sample stream into a collection vessel. Compared to droplet cell sorters, fluid switching cell sorters have a lower maximum cell sorting rate due to the cycle time of the mechanical system used to divert the sample stream. This cycle time, the time between initial sample diversion and when stable non-sorted flow is restored, is typically significantly greater than the period of a droplet generator on a droplet cell sorter. This longer cycle time limits fluid switching cell sorters to processing rates of several hundred cells per second. For the same reason, the stream segment switched by a fluid cell sorter is usually at least ten times the volume of a single micro-drop from a droplet generator. This results in a correspondingly lower concentration of cells in the fluid switching sorter's collection vessel as compared to a droplet sorter's collection vessel.
Newer generation microfluidics technologies offer great promise for improving the efficiency of fluid switching devices and providing cell sorting capability on a chip similar in concept to an electronic integrated circuit. Many microfluidic systems have been demonstrated that can successfully sort cells from heterogeneous cell populations. They have the advantages of being completely self-contained, easy to sterilize, and can be manufactured on sufficient scales (with the resulting manufacturing efficiencies) to be considered a disposable part.
A generic microfluidic device is schematically illustrated in FIG. 1 and indicated generally at 10. The microfluidic device 10 comprises a substrate 12 having a fluid flow channel 14 formed therein by any convenient process as is known in the art. The substrate 12 may be formed from glass, plastic or any other convenient material, and may be substantially transparent or substantially transparent in a portion thereof. The substrate 12 further has three ports 16, 18 and 20 coupled thereto. Port 16 is an inlet port for a sheath fluid. Port 16 has a central axial passage that is in fluid communication with a fluid flow channel 22 that joins fluid flow channel 14 such that sheath fluid entering port 16 from an external supply (not shown) will enter fluid flow channel 22 and then flow into fluid flow channel 14. The sheath fluid supply may be attached to the port 16 by any convenient coupling mechanism as is known to those skilled in the art.
Port 18 also has a central axial passage that is in fluid communication with a fluid flow channel 14 through a sample injection tube 24. Sample injection tube 24 is positioned to be coaxial with the longitudinal axis of the fluid flow channel 14. Injection of a liquid sample of cells into port 18 while sheath fluid is being injected into port 16 will therefore result in the cells flowing through fluid flow channel 14 surrounded by the sheath fluid. The dimensions and configuration of the fluid flow channels 14 and 22, as well as the sample injection tube 24 are chosen so that the sheath/sample fluid will exhibit laminar flow as it travels through the device 10, as is known in the art. Port 20 is coupled to the terminal end of the fluid flow channel 14 so that the sheath/sample fluid may be removed from the microfluidic device 10.
While the sheath/sample fluid is flowing through the fluid flow channel 14, it may be analyzed using cytometry techniques by shining an illumination source through the substrate 12 and into the fluid flow channel 14 at some point between the sample injection tube 24 and the outlet port 20. Additionally, the microfluidic device 10 could be modified to provide for a cell sorting operation, as is known in the art.
Although basic microfluidic devices similar to that described hereinabove have been demonstrated to work well, there is a need in the prior art for improvements to cytometry systems employing microfluidic devices. The present invention is directed to meeting this need.